In my experience design revamps of this magnitude has always needed fresh markup, CSS, Javascript - aka an entirely new front-end. But in case it is just the color of all buttons, then you'd change them in the components. 700 components is quite a stretch - for example there are at most 50 components in Morning Star's design system - http://designsystem.morningstar.com/.
But let's say there are 700 components, what's the alternative if we were using BEM? You'd have to change all the 700 CSS classes, breaking things everywhere thanks to cascades, inheritance, multiple selectors, and specificity. With Functional CSS, all you have to do is go to the markup, and change "bg-white" to "bg-offgrey", and nothing breaks.
But let's say there are 700 components, what's the alternative if we were using BEM? You'd have to change all the 700 CSS classes, breaking things everywhere thanks to cascades, inheritance, multiple selectors, and specificity. With Functional CSS, all you have to do is go to the markup, and change "bg-white" to "bg-offgrey", and nothing breaks.